A phagocyte is a generic concept for cells having phagocytic activity. A phagocyte is broadly divided into professional phagocytes such as neutrophils, macrophages, or dendritic cells whose main function is phagocytosis in the living body, and amateur phagocytes which occasionally exhibit phagocytic activity under a certain set of conditions. Exemplary amateur phagocytes include microglia cells of brain, Kupffer cells of liver, and Sertoli cells of testis.
A leukocyte is a cell component of blood, and the leukocyte is mainly constituted from granulocytes (about 60%), lymphocytes (about 25%), and monocytes (about 5%). Granulocytes can be further divided into neutrophils, eosinophils, and basophils. The neutrophils outnumber other types of the leukocyte, and it constitute about half of the leukocyte. The monocytes migrate through various tissues to become macrophages. Since a leukocyte includes many phagocytes and they can be isolated therefrom with ease, it is a preferable source of phagocytes.
Phagocytosis is one of the most important defense mechanisms that start at the earliest timing against invasion by microorganisms such as bacteria or fungi. Phagocytosis proceeds through sequential steps of recognizing a foreign substance by a phagocyte, intaking the foreign substance, forming a phagosome, digesting the foreign substance in the phagosome, and absorption or excretion of the digested substance. Phagocytosis occurs not only for foreign substances such as bacteria or fungi, but also on autologous substances no longer needed in the body such as residue of the autologous tissue at the site of inflammation and waste autologous cell. A phagocyte does not only digest the engulfed substances in the cell, but in some cases, it releases oxygen radicals and proteases to the exterior of the cell during the phagocytosis. While this may enable efficient local disinfection and tissue digestion, the excessive phagocytosis may result in the destruction of the autologous tissue. For example, in the lesion of rheumatoid arthritis (RA), cells which have excessively engulfed the autoimmune complex (RA cell) are found, and in such lesion, the tissue is damaged by the protease released from the RA cell, and this contributes for the progress of the arthritis.
Methods known for assaying the phagocytic function of a phagocyte include the method in which the phagocyte is brought in touch with latex particles and the latex particles engulfed in the cell is counted by means of cytometry or microscopic observation, the method in which the phagocyte is allowed to engulf fluorescence-labeled substance (E. coli, zymosan, and the like), and the amount of the substance engulfed by the phagocyte is detected, the method in which number of viable bacteria is confirmed by cultivation after the phagocytosis of the living bacteria, and the method in which luminescence from oxygen radical during the phagocytosis is detected.
CD14 molecule is a glycoprotein which has been identified as a differentiation maker expressed on the monocyte membrane, and it is known to have the function of a receptor for LPS (lipopolysaccharide) (Non-Patent Literature 1). Known molecular species of the CD14 molecule include two types, namely, membrane-bound CD14 (mCD14) which is expressed on the cell surface and soluble CD14 (sCD14). Known sCD14 molecular species include the one having a molecular weight of about 55 kDa and the one having a molecular weight of about 49 kDa and these are believed to be produced by secretion from liver as well as cleavage by mCD14 enzyme associated by the activation of a monocyte (Non-Patent Literatures 2 to 4).
The sCD14 having a molecular weight of about 55 kDa and the one having a molecular weight of about 49 kDa (hereinafter referred to “high molecular weight sCD14”) are reported to increase in the blood of patients suffering from sepsis, acquired immunodeficiency syndrome (AIDS), acute respiratory distress syndrome (ARDS), systemic lupus erythematosus (SLE), and many other diseases. Accordingly, these sCD14 are not considered to be disease-specific markers (Non-Patent Literatures 5 to 6).
In the meanwhile, sCD14-ST (soluble CD14 antigen subtype) has been reported as a new sCD14 molecular species which shows characteristic increase in the blood concentration in sepsis patients.
sCD14-ST is a sCD14 which has a molecular weight of 13±2 kDa when electrophoresed under non-reducing conditions on SDS-PAGE, and which retains N terminal region of the CD14. Compared to the high molecular weight sCD14, sCD14-ST lacks extensive region on the C terminal side of the amino acid sequence, and it does not have LPS binding ability that the high molecular weight sCD14 has. sCD14-ST also exhibits an immunogenicity different from that of the high molecular weight sCD14, and it can be differentiated from the high molecular weight sCD14 by using antibodies. Blood concentration of the sCD14-ST is specifically high in sepsis patients (Patent Literature 1). It has also been reported that the blood concentration of the sCD14-ST is high in the sepsis patients compared to patients of systemic inflammatory response syndrome (SIRS), which is a disease whose differentiation from sepsis has been difficult. Accordingly, sCD14-ST has been considered to be a specific diagnosis marker of sepsis (Non-Patent Literature 7).